Whispers from the dark side: What can gravitational waves reveal about dark matter? Scientists of the PRISMA+ Cluster of Excellence analyze data from the gravitational wave observatory NANOGrav

Representative illustration of the Earth embedded in space-time which is deformed by the background gravitational waves and its effects on radio signals coming from observed pulsars .  Credit: Ill./©: NANOGrav / T. Klein. Hi-res image

Scientists of the PRISMA⁺ Cluster of Excellence analyze data from the gravitational wave observatory NANOGrav

The NANOGrav Collaboration recently captured the first signs of very low-frequency gravitational waves. Prof. Pedro Schwaller and Wolfram Ratzinger analyzed the data and, in particular, considered the possibility of whether this may point towards new physics beyond the Standard Model. In an article published in the journal SciPost Physics, they report that the signal is consistent with both a phase transition in the early universe and the presence of a field of extremely light axion-like particles (ALPs). The latter are considered as promising candidates for dark matter.

Gravitational waves open a window into the early universe. While the ubiquitous cosmic microwave background yields no clues about the first 300,000 years of our universe, they provide some glimpses of what happened during Big Bang. "It's exactly this very early universe that is so exciting for particle physicists," explains Pedro Schwaller, Professor of Theoretical Physics at the PRISMA⁺ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU). "This is the time when the elementary particles like quarks and gluons are present, and then combine to form the building blocks of atomic nuclei."

The special thing about the gravitational waves which the NANOGrav Collaboration has detected for the first time is that they have a very low frequency of 10-8 Hertz, which equates to approximately one oscillation per year. Due to their correspondingly large wavelength, in order to detect them any detector would also have to be equally large. As such a detector is not possible here on Earth, the astronomers at NANOGrav use distant pulsars and their light signals as huge detectors.

Wolfram Ratzinger outlines the motivation behind their work: "Even though so far the data only provides us with a first hint of the existence of low-frequency gravitational waves, it is still very exciting for us to work with them. This is because such waves could be produced by various processes that occurred in the early universe. We can now use the data we already have to decide, which of these come into consideration and which do not fit the data at all."

As a result, the Mainz-based scientists decided to take a particularly close look at two scenarios that could have caused the observed gravitational waves: Phase transitions in the early universe and a dark matter field of extremely light axion-like particles (ALPs). Phase transitions such as these occur due to the falling temperature in the primordial soup after the Big Bang and result in massive turbulences – however, like dark matter they are not covered by the Standard Model.

Based on the data available, Pedro Schwaller and Wolfram Ratzinger interpret the results of their analysis with relative caution: "Perhaps slightly more probable is the early phase transition scenario." On the other hand, the two physicists believe that the fact that they are able to work out certain possibilities based only on limited data proves the potential of their approach. "Our work is a first, but important development – it gives us a lot of confidence that with more precise data we can draw reliable conclusions about the message gravitational waves are sending us from the early universe."

"Furthermore," Pedro Schwaller concludes, "we can already begin to pin down certain characteristics of the scenarios and put constraints on them, in our case the strength of the phase transition and the mass of the axions." 

Publication

W. Ratzinger, P. Schwaller, Whispers from the dark side: Confronting light new physics with NANOGrav data, SciPost Physics 10, 23 February 2021,
DOI:10.21468/SciPostPhys.10.2.047

Related Links

• PRISMA⁺ Cluster of Excellence
• Theoretical High Energy Physics (THEP) at the Institute of Physics

Contact:

Prof. Dr. Pedro Schwaller
Theoretical High Energy Physics (THEP)
and PRISMA+ Cluster of Excellence
Institute of Physics 
Johannes Gutenberg-Universität Mainz
55099 Mainz
E-mail | Homepage

Wolfram Ratzinger
Theoretical High Energy Physics (THEP)
and PRISMA+ Cluster of Excellence
Institute of Physics 
Johannes Gutenberg-Universität Mainz
55099 Mainz
E-mail

 Source: Johannes Gutenberg-Universität Mainz/Press Release

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Hunting the Original Star Stuff

X-ray light pulses out in rings from a neutron star in the Circinus X-1 star system. 

Credit: NASA's Chandra X-ray Observatory

SDSU astrophysicist Fridolin Weber will present his research on “Big Bang matter” at this year’s Albert W. Johnson lecture.

Every atom in every molecule of your body was born in a single spectacular, 2000-billion-degree Kelvin explosion some 13.8 billion years ago. But the Big Bang also produced exotic forms of matter that lasted only fleeting seconds before blinking out of existence. Fridolin Weber searches the universe for these elusive particles that can only exist in extreme astronomical conditions, such as inside the hearts of super-dense neutron stars. The San Diego State University theoretical astrophysicist will present findings from his galactic hunt on Friday, April 7, at the annual Albert W. Johnson Lecture.

“It’s kind of mind-boggling. If things had happened just a little differently in the early universe, we wouldn’t be here.” 

Weber’s quarry is the quark, an elementary particle that constitutes matter’s most fundamental building block. Quarks are bound up in composite particles like protons and neutrons and are generally not found in nature by themselves. The exception is inside neutron stars, which are incredibly dense remnants of massive stars blown apart by supernova explosions. Composed primarily of neutrons, they are only 24 kilometers (15 miles) or so in diameter, yet are twice as massive as our sun.

That amount of mass packed into a relatively miniscule area creates extraordinary density at the star’s core, squeezing atomic nuclei so tightly that fundamental particles like quarks can exist freely. It’s the closest parallel to conditions immediately after the Big Bang that we know of in our universe.

“We want to understand what happened in the moments and minutes after that gigantic explosion,” Weber said. “We turn to neutron stars to see if we can detect the astrophysical signature of this ‘Big Bang matter.’”

Weber and his colleagues trawl data from enormous radio telescopes scattered around the world. They’re looking for distortions in radio waves emitted by stars that are characteristic of neutron stars’ unusually high temperatures. Right now, astrophysicists know of about 2,000 neutron stars in the sky, but Weber expects that number to grow to more than 30,000 in the coming years as telescopes and computing technology improve.

Just because you’ve found a neutron star doesn’t mean you’ve found quarks, though. Once a good candidate is located, Weber looks for a secondary pattern.

A neutron star is a magnetically charged sphere that radiates energy over time, causing it to “spin down,” like a spinning figure skater with outstretched arms. At the same time, the star is becoming denser and denser. Finally, the theory goes, the density will become so great that the atomic nuclei within the star’s core will break apart, forming quarks. This briefly makes the star “spin up” again—the figure skater pulling in her arms—before the quarks dissipate and the star resumes spinning down. Astrophysicists like Weber can detect this “spin down, spin up, then spin down again” pattern, allowing them to indirectly rewind the universe to its very beginning.

“These quarks would exist as plasma, which would have existed in the first couple of minutes after the Big Bang,” he said.

It’s easy to get lost in the fine-grained data and details needed to study complex astrophysics, but when Weber steps back from all that and considers the connection every single molecule in the universe shares with that single celestial moment, he’s humbled.

“It’s kind of mind-boggling,” he said. “If things had happened just a little differently in the early universe, we wouldn’t be here."

Weber’s lecture, “Searching for Big Bang Matter in Stars,” will take place at 3 p.m. on Friday in Storm Hall West, Room 11. For more information, visit the Albert W. Johnson University Research Lectureship website.  

Source: San Diego State University (SDSU)/News Center